The present application relates to the field of imaging apparatuses. It finds particular application to imaging apparatuses that employ radiation (e.g., x-rays, gamma-rays, etc.) to image an object. For example, medical, security, and/or industrial applications may utilize a computed tomography (CT) apparatus to examine an object. Based upon the attenuation experienced by radiation traversing the object, one or more images of the object may be generated from the examination.
Today, CT and other imaging apparatuses that employ radiation technology (e.g., single-photon emission computed tomography (SPECT), mammography, digital radiography, etc.) are useful to provide information, or images, of interior aspects of an object under examination. Generally, the object is exposed to radiation comprising photons (e.g., such as x-rays, gamma rays, etc.), and an image(s) is formed based upon the radiation absorbed and/or attenuated by the interior aspects of the object, or rather an amount of photons that is able to pass through the object. Generally, highly dense aspects of the object absorb and/or attenuate more radiation than less dense aspects, and thus an aspect having a higher density, such as a bone or metal, for example, will be apparent when surrounded by less dense aspects, such as muscle or clothing.
Imaging apparatuses that employ radiation technology generally comprise, among other things, a detector array having of a plurality of cells respectively configured to convert radiation that has traversed the object into signals that may be processed to produce the image(s). The cells are typically “energy integrating” or “photon counting” type cells (e.g., the imaging apparatus operates in energy integrating mode or photon counting mode).
Energy integrating cells are configured to convert energy into signals (e.g., current or voltage signals) that are proportional to an incoming photon flux rate and a photon energy. That is, charge collected in respective cells is integrated over a time period (e.g., at times referred to as a measurement interval), sampled, and digitized. While this type of cell is widely used, there are several drawbacks to energy integrating cells. For example, energy integrating cells are generally not able to provide feedback as to the number and/or energy of photons detected. As another drawback, there is a lower limit of detection defined by noise such that a cell with little to no incident radiation may produce some signal due to thermal and/or analog read noise (e.g., produced by the detector array and/or readout components). It may be appreciated that as a result of this lower limit, the dose of radiation that is applied to an object under examination is generally greater than the dose of radiation that may be applied to the object if the cells are of a photon counting type.
Photon counting cells are configured to output signals for respective detection events and may be configured to convert energy into signals that are proportional to the energy of a detected photon (e.g., at times referred to as a detection event). Thus, ideally, signals produced by respective cells generally comprise one or more current and/or voltage pulses, for example, respectively associated with a single detection event. By way of example, in one embodiment an output signal is proportional to the energy of a detected photon and in anther embodiment an output signal merely corresponds to a photon count. A controller may then be used to determine the location and energy of respective detection events, accumulate the detection events occurring during a measurement interval (e.g., an “acquisition view”), digitize the information, and/or process the digital information to form an image, for example. It may be appreciated that there are numerous advantages to photon counting cells over energy integrating cells. For example, the counting of photons is essentially noise free (e.g., apart from inherent photon shot noise). Therefore, a lower dose of radiation may be applied to the object under examination. Moreover, photon counting cells generally allow for energy or wavelength discrimination. Therefore, images resulting from radiation emitted at different energy levels may be obtained at the same or substantially the same time, for example.
While photon counting detector arrays (e.g., detector arrays comprising photon counting cells) have numerous advantages over energy integrating detector arrays, photon counting detector arrays have not been widely applied in some imaging modalities (e.g., such as in CT apparatuses) due to cost considerations and other challenges associated with photon counting detector arrays. For example, CT systems generally have a high flux rate, which may cause saturation issues (e.g., pulse pileup) in photon counting detector arrays because photon counting cells may be unable to return to a normal state after the detection of a photon and before another photon is detected.